1. Field of the Invention
The present invention relates to a frequency generator and a communication system using the same. More specifically, the present invention relates to temperature compensation of an oscillation frequency of a frequency generator.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing an example of a differential LC resonance type of voltage controlled frequency generator using a prior art LC resonator. The differential LC resonance type of voltage controlled frequency generator has a differential amplifier 30 having an LC resonator 10 as a load and feedback buffer circuits 21, 22 having transistors Q3, Q5 feeding back an output signal of the differential amplifier to its input. The buffer circuit receives as an input an output of the collector of one of transistors Q11, Q12 forming a differential transistor pair of the amplifier and inputs an output to the base of the other transistor. Typically, such kind of feedback buffer circuit desirably feeds back a signal without attenuating it. A circuit construction such as a capacitor, emitter follower circuit or source follower circuit is generally employed.
An oscillation frequency of the differential LC resonance type of voltage controlled frequency generator is expressed by the following equation (1) using inductance L of inductors L1, L2 constructing the LC resonator 10, capacitance CV of diodes C1, C2, and wiring parasitic capacitance CP:
                              f          OSC                =                  1                      2            ⁢            π            ⁢                                          L                ⁡                                  (                                                            C                      V                                        +                                          C                      P                                                        )                                                                                        (        1        )            
Frequency control is performed by controlling the capacitance CV of the diodes C1, C2 as variable capacitors with a voltage applied to frequency control terminal VCONT. When the buffer circuits 21, 22 are constructed by capacitive coupling, to form capacitance used in capacitance feedback in an integrated circuit, capacitance between metal wire layers, capacitance between polycrystalline silicon layers, or capacitance between a polycrystalline silicon and a metal wire layer is generally used.
As shown in FIG. 1, when using the emitter follower circuit as the feedback buffer circuit, wiring parasitic capacitance parasitized in signal feedback capacitance in capacitance feedback can be reduced. When an oscillation frequency is high, a wide frequency tuning range can be obtained. As an oscillation frequency is increased, it is effective to use the emitter follower circuit as a feedback circuit. Since the emitter follower circuit has a low output impedance, the transistors Q11, Q12 constructing the differential amplifier 30 can be easily driven and the oscillator can be oscillated at high frequencies.
The construction as shown in FIG. 1 is disclosed in Japanese Patent Application Laid-Open No. 2001-156545.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2001-156545 (FIG. 3)
[Non-Patent Document 1]
Written by Sima Dimitrijev, “Understanding Semiconductor Devices”, Oxford University Press, 2000, pp. 168–170
[Non-Patent Document 2]
Written by Allen and Holberg, “CMOS Analog Circuit Design”, Oxford University Press, 1987, pp. 124–127